The present invention relates to an automatic focusing apparatus, and more particularly to an imaging apparatus having an automatic focusing function capable of carrying out automatic focusing With a stable detection accuracy.
FIG. 1 shows a schematic block diagram of a conventional automatic focusing imaging apparatus, and FIGS. 2A and 2B show diagrams for explaining a focal point voltage. The following is a description with reference to the drawings.
In FIG. 1, imaging means 1 images an image pick-up light 1i through a lens system on a photoelectric conversion plane of a photoelectric conversion element to obtain an image signal 1a, and supplies the image signal 1a to a signal processing circuit 2 which amplifies the image signal 1a and adds a synchronous signal such as a vertical blanking signal to the image signal to obtain a video signal to supply an edge extraction circuit 3 and a recording circuit (not shown). The edge extraction circuit 3 extracts a high frequency band component in the image signal 1a and supplies to a logarithmic conversion circuit 4 each field of a focal point voltage E which is obtained after a detected voltage is converted from analog to digital.
Here, there is described a focal point voltage E representing an extent of an in-focus position in accordance with FIGS. 2A and 2B. When the apparatus photographs a normal picture image having a plurality of edges, the focal point voltage E has characteristics which are shaped like a hanging bell and can be approximated by an equation (1) as follows: EQU E(X)=p.multidot.EXP[-qX.sup.2 ] (1),
where p and q are constants.
A logarithm conversion circuit 4 performs a logarithm conversion for the focal point voltage E to output a logarithm conversion signal 4a. When a voluntary field is n, the logarithm conversion signal 4a is represented by an equation as follows: EQU Log [E(X)]=Log p-q.times.X.sup.2 ( 2)
Then, a calculation circuit 5 to which the logarithm conversion signals 4a are supplied, compares the signals 4a in an n field and an "n+1" field to calculate a moving amount X of the lens at the position having the maximum value E.sub.max of the focal point voltage E in accordance with an equation (3) as follows: EQU X=E.sup.(1) (X)/E.sup.(2) (X) (3)
The calculation circuit 5 supplies to a drive circuit 6 a drive circuit control signal 5a which is obtained as a calculation result on the basis of the moving amount X of the lens. The drive circuit 6 causes the lens which is mounted in the imaging means 1 and not shown in the figure to be in focus, and therefore, the conventional imaging apparatus performs the automatic focusing operation.
In the conventional technology described above, the focal point voltage E has the characteristics shown in FIG. 2A, assuming the case that a normal picture signal including a plurality of frequency components is imaged, and automatic focusing operation is performed on the basis of the logarithm conversion signal 4a which is obtained by logarithm conversion in the circuit 4.
However, when the image signal only including a specific frequency component is recorded, the focal point voltage E does not always have the characteristics shown in FIG. 2. For example, when the apparatus images vertical stripes having the same intervals, the focal point voltage E(X) has the characteristics shown in FIG. 2B, and cannot be approximated by the equation (1), and so it is impossible to precisely perform automatic focusing operation.